GETSOCKOPT(2) BSD GETSOCKOPT(2)
NAME
getsockopt, setsockopt - get and set options on sockets
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
getsockopt(s, level, optname, optval, optlen)
int s, level, optname;
char *optval;
int *optlen;
setsockopt(s, level, optname, optval, optlen)
int s, level, optname;
char *optval;
int optlen;
DESCRIPTION
getsockopt and setsockopt manipulate "options" associated with a socket.
Options may exist at multiple protocol levels; they are always present at
the uppermost "socket" level.
When manipulating socket options, the level at which the option resides
and the name of the option must be specified. To manipulate options at
the socket level, level is specified as SOL_SOCKET. To manipulate
options at any other level, the protocol number of the appropriate
protocol controlling the option is supplied. For example, to indicate
that an option is to be interpreted by the TCP protocol, level should be
set to the protocol number of TCP; see getprotoent(3N).
The parameters optval and optlen are used to access option values for
setsockopt. For getsockopt, they identify a buffer in which the value
for the requested option(s) is to be returned. For getsockopt, optlen is
a value-result parameter, initially containing the size of the buffer
pointed to by optval, and modified on return to indicate the actual size
of the value returned. If no option value is to be supplied or returned,
optval may be supplied as 0.
optname and any specified options are passed uninterpreted to the
appropriate protocol module for interpretation. The include file
<sys/socket.h> contains definitions for socket-level options, described
below. Options at other protocol levels vary in format and name; consult
the appropriate entries in section (4P).
Most socket-level options take an int parameter for optval. For
setsockopt, the parameter should nonzero to enable a Boolean option, or 0
if the option is to be disabled. SO_LINGER uses a struct linger
parameter, defined in <sys/socket.h>, which specifies the desired state
of the option and the linger interval (see below).
The following options are recognized at the socket level. Except as
noted, each may be examined with getsockopt and set with setsockopt.
SO_DEBUG Toggle recording of debugging information.
SO_REUSEADDR Toggle local address reuse.
SO_KEEPALIVE Toggle keep connections alive.
SO_DONTROUTE Toggle routing bypass for outgoing messages.
SO_LINGER Linger on close if data present.
SO_BROADCAST Toggle permission to transmit broadcast messages.
SO_OOBINLINE Toggle reception of out-of-band data in band.
SO_SNDBUF Set buffer size for output.
SO_RCVBUF Set buffer size for input.
SO_TYPE Get the type of the socket (get only).
SO_ERROR Get and clear error on the socket (get only).
SO_DEBUG enables debugging in the underlying protocol modules.
SO_REUSEADDR indicates that the rules used in validating addresses
supplied in a bind(2) call should allow reuse of local addresses.
SO_KEEPALIVE enables the periodic transmission of messages on a connected
socket. Should the connected party fail to respond to these messages,
the connection is considered broken and processes using the socket are
notified via a SIGPIPE signal. SO_DONTROUTE indicates that outgoing
messages should bypass the standard routing facilities. Instead,
messages are directed to the appropriate network interface according to
the network portion of the destination address.
SO_LINGER controls the action taken when unsent messags are queued on
socket and a close(2) is performed. If the socket promises reliable
delivery of data and SO_LINGER is set, the system will block the process
on the close attempt until it is able to transmit the data or until it
decides it is unable to deliver the information (a time-out period,
termed the "linger" interval, is specified in the setsockopt call when
SO_LINGER is requested). If SO_LINGER is disabled and a close is issued,
the system will process the close in a manner that allows the process to
continue as quickly as possible.
The option SO_BROADCAST requests permission to send broadcast datagrams
on the socket. Broadcast was a privileged operation in earlier versions
of the system. With protocols that support out-of-band data, the
SO_OOBINLINE option requests that out-of-band data be placed in the
normal data input queue as received; it will then be accessible with recv
or read calls without the MSG_OOB flag. SO_SNDBUF and SO_RCVBUF are
options to adjust the normal buffer sizes allocated for output and input
buffers, respectively. The buffer size can be increased for high-volume
connections or decreased to limit the possible backlog of incoming data.
The system places an absolute limit on these values. Finally, SO_TYPE
and SO_ERROR are options used only with setsockopt. SO_TYPE returns the
type of the socket, such as SOCK_STREAM; it is useful for servers that
inherit sockets on startup. SO_ERROR returns any pending error on the
socket and clears the error status. It can be used to check for
asynchronous errors on connected datagram sockets or for other
asynchronous errors.
ERRORS
The call succeeds unless any of the following are true:
[EBADF] The argument s is not a valid descriptor.
[ENOTSOCK] The argument s is a file, not a socket.
[ENOPROTOOPT] The option is unknown at the level indicated.
[EFAULT] The address pointed to by optval is not in a valid part
of the process address space. For getsockopt, this error
may also be returned if optlen is not in a valid part of
the process address space.
SEE ALSO
ioctl(2), socket(2), getprotoent(3N)
DIAGNOSTICS
A 0 is returned if the call succeeds, -1 if it fails.
BUGS
Several of the socket options should be handled at lower levels of the
system.